1. Field of the Invention
The present invention relates to a paper feeder for use in an image forming apparatus which forms images on printing paper and more particularly to a paper feeder for separating and feeding sheets of printing paper placed on a paper feeding stand.
2. Description of the Belated Art
FIG. 6 is a side view of an image forming apparatus, showing a stencil printer. A stencil obtained by a thermal process is affixed to the outer periphery of a drum 50 which constitutes a printing portion, and printing paper P is fed continuously between the drum 50 and a press roller 50a to effect printing in accordance with images on the stencil.
Sheets of printing paper P, which stacked on a paper feeding stand 51, are picked up one by one successively from the top and fed toward the drum 50 by means of a scraper roller 52 and a pickup roller 53, both constituting a primary paper feeding mechanism 57. On the side opposed to the pickup roller 53 are disposed a separator means and an overlap detecting means (neither shown) to separate the paper sheets one by one without overlapping and feed them toward a secondary paper feeding mechanism 58.
Between the drum 50 and the pickup roller 53 is disposed a timing roller 60 which constitutes the secondary paper feeding mechanism 58, and by operation of the timing roller 60, each sheet of printing paper P is fed toward the drum at a predetermined timing which is synchronized to the rotation of the drum 50, and printing is performed.
Thus, the stencil printer is provided with the primary and secondary paper feeding mechanisms 57, 58. The reason why the stencil printer has such two stages of paper feeding mechanisms is that the stencil printer operates at a higher speed in comparison with printers at large, e.g. copying apparatus The primary paper feeding mechanism 57 functions to take out printing sheets P one by one from the paper feeding stand 51 positively without overlapping at a relatively low speed, and the secondary paper feeding mechanism 58 functions to feed the printing sheets P thus taken out to the drum 50 at a high speed corresponding to the rotational speed of the drum which is rotating at a high speed.
In the stencil printer, the printing speed is variable and can be adjusted stepwise from low (e.g. 60 rpm) to high (e.g. 130 rpm).
A belt 66 is stretched among the drum 50 and a plurality of idlers 55 as well as a rotating shaft 65a of a motor 65, and by operation of the motor 65, the drum 50 is rotated. The primary and secondary paper feeding mechanisms 57, 58 also utilize the driving force of the motor 65.
The secondary paper feeding mechanism 58 is provided with an intermittent motion mechanism 70. In the intermittent motion mechanism 70, a guide roller cam 71 and a timing roll cam 72 are fixed to the rotating shaft 65a of the motor 65, one end 73a of a generally sectorial gear 73 is in abutment with the guide roller cam 71, and one end 74a of a timing lever 74 is in abutment with the timing roll cam 72.
With rotation of the guide roller cam 71, the sectorial gear 73 moves pivotally about a shaft 73c to rotate a guide roller 75 in forward and reverse directions successively, the guide roller 75 being engaged with an opposite end 73b of the gear 73.
As the timing roll cam 72 rotates, an opposite end 74b of the timing lever 74 moves pivotally in an intermittent manner and urges the timing roller 60 toward the guide roller 75.
When the guide roller 75 rotates in the forward direction, the timing roller 60 is urged toward the guide roller 75 to feed each printing sheet P toward the drum 50 at a predetermined timing.
In the above conventional apparatus, the primary and secondary paper feeding mechanisms 57, 58 are of a merely mechanically coupled construction wherein they are driven by the motor 65 through the belt 66, so when the number of revolutions of the motor 65 is changed upon change in the printing speed, the number of revolutions of the pickup roller 53 in the primary paper feeding mechanism 57 is also changed.
For example, as the printing speed increases, the pickup roller 53 also rotates at a correspondingly high speed, resulting in that it is no longer possible to separate the printing sheets P. More particularly, while the rotating speed of the pickup roller 53 is high, the friction between the roller 53 and the printing sheet P is kept in the state of kinetic friction, so that the pickup roller runs idle with respect to the sheets P and it is impossible to take out the sheets from the paper feeding stand 51.
Thus, a limit is encountered in the speed-up of printing in the image forming apparatus.
As the printing speed decreases, the pickup roller 53 also rotates at a low speed and can separate the printing sheets P and deliver them to the secondary paper feeding mechanism 58. In this case, however, the printing sheets P are apt to overlap each other. The conventional paper feeding mechanism is not provided with means capable of surely preventing the occurrence of such overlapping of printing sheets, although there may be used an overlap detecting means for detecting such overlapping after occurrence.
In both cases mentioned above, it is necessary to prevent the occurrence of the foregoing problems, and particularly it has been desired to provide means which is applicable also to the case where the printing speed is variable.
The present invention has been accomplished in view of the above-mentioned problems and it is the object of the invention to provide a paper feeder capable of separating and feeding printing sheets always stably even upon change in the printing speed in an image forming apparatus of the type wherein the printing speed is variable, and also capable of being constructed easily.